1. Field of the Invention
The invention relates to an ultrahigh pressure mercury lamp, and especially to an ultrahigh pressure mercury lamp which is used as the light source for a projection device of the projection type, such as a liquid crystal projector or the like.
2. Description of the Related Art
In a light source which is used for a liquid crystal projector or the like, the emitted light is projected by means of a reflector in one direction and is emitted onto a screen by an optical system, such as a lens and the like. It is desirable for this light source to be as nearly as possible a point light source. However, in practice, there is a certain size which is determined by the distance between the electrodes of the lamp which is the light source. If the size of this light source is considered to be approximately a point light source, the lamp can be imagined as an ideal lamp in which the bulb part has a uniform thickness and is made spherical and in which the middle of the arc which is formed within this bulb by a discharge that is located in the center of the bulb part.
However, in the case, for example, of an ultrahigh pressure mercury lamp which is driven using a direct current, the sizes of the anode and the cathode which are located in the bulb of this lamp differ largely from one another. This is because, in the case of driving using a direct current, the amount of heat which is formed in the respective electrode is to a large extent varied. Therefore, the anode is made larger than the cathode with consideration of this amount of heat. In order to place these electrodes within a discharge vessel, differently than in the case of the above described ideal lamp, for example, the measure described in Japanese patent disclosure document HEI 11-111226 that the bulb part is made essentially as an ovoid, or other measures are taken.
FIG. 10 shows a conventional ultrahigh pressure mercury lamp in which the bulb part is essentially an ovoid of circular cross section. A bulb part 51 is made of a translucent material, such a silica glass or the like. Extending from opposite ends of the bulb part 51 are side tube parts 52. The bulb part 51 and the side tube parts 52 form a discharge vessel 50 in which an anode electrode 53 is disposed opposite a cathode 54 electrode. Each of the anode and cathode electrodes 53, 54 is welded to an end of a respective metal foil 55 made of molybdenum or the like. An outer lead 56 is welded on the other end of each metal foil 55. The inside of this discharge vessel 50 is an ovoid as was described above. Furthermore, in addition to anode and cathode electrodes 53, 54, this discharge vessel 50 is filled with a rare gas and mercury in an amount of roughly 0.15 mg/mm3. Additionally, an arrangement is made in which the middle of the arc which forms between the anode and cathode electrodes 53, 54 coincides with the middle of the bulb part 51, at which the maximum diameter of the bulb part 51 is located. The distance between the electrodes is, for example, 1.5 mm.
There is market demand for further increasing the radiance of this lamp. The improvement has been made that, by shortening the distance between the electrodes of this lamp, the input wattage per unit of distance between the electrodes is increased and thus the radiance is increased, or that the diameter of the arc is reduced by further increasing the amount of mercury to be added to the discharge vessel and that the radiance is thus increased.
However, in the case in which the amount of mercury to be added to the discharge vessel has been increased even more, when for example roughly 0.17 mg/mm3 are added, the mercury in the vicinity of the base point on the cathode side in the discharge space is not yet vaporized. Therefore, the failure of the mercury to be vaporized has been corrected by the middle position of the arc being pushed out of the area with the maximum diameter of the bulb part which is the middle of the bulb part, towards the cathode side. With this measure the not yet vaporized mercury is thus heated and caused to vaporize by the arc as the heat source approaching the vicinity of the base point of the cathode.
With respect to the radiation intensity which is required by the market, there is a demand for a lamp with a greater radiance and good color reproduction. Furthermore, there is a demand for making the lamp itself smaller. However, if the amount of mercury to be added to the conventional lamp is increased even more and is fixed at least 0.2 mg/mm3, the disadvantage occurs that in the remaining area, for example, at the base point of the anode, the mercury fails to vaporize, even if the position of the arc in the discharge vessel is shifted to the cathode side. Additionally, there is also the disadvantage that the arc becomes unstable and flickering occurs. This is caused by the following:
The mercury which has not vaporized collects and contracts. If the grain size of this mercury reaches a certain magnitude, especially roughly 0.2 mm or more, a cycle forms in which the mercury is moved by gravity to the area with the maximum inside diameter and vaporizes and then mercury condenses again on the base point of the anode. For this reason, convection within the lamp fluctuates.
If, by shortening the distance between the electrodes, the attempt is made to obtain high radiation intensity, the amount of heat flowing into these electrodes is large, and especially the wear on the anode is very great, resulting in the disadvantage that the service life of the lamp is shortened. On the other hand, it can be imagined that the volume of the anode itself increases in order to suppress the heat influence on the anode. However, there was the disadvantage that by increasing the diameter of the anode part, the diameter of the bulb part increases and that the demand for reducing the size of the lamp cannot be satisfied.